The National Highway Traffic Safety Administration has found a potentially "catastrophic" defect in as many as 5.3 million General Motors Corp. cars and sedan pickups produced between 1978 and 1980, the agency announced yesterday.

NHTSA said the defect could lead to the possible separation of the rear axle shaft and wheel assembly, "which could lead to loss of control, accidents, injuries, death or property damage."

The agency said it is "aware of 64 alleged accidents and 11 injuries" involving axle-shaft failure.

Affected cars and sedan pickups include the 1978-1980 models of the Chevrolet Malibu, Monte Carlo and El Camino; the Pontiac LeMans and Grand Prix; the Oldsmobile Cutlass and Cutlass Supreme; the Buick Century and Regal, and the GMC Caballero.

NHTSA called its finding "an initial determination of defect," based on an agency investigation that began last spring. The finding is the first step in a process that could lead to a recall.

NHTSA officials said the agency would hold a public hearing on the finding on May 4, beginning at 10 a.m., in Room 2230 of the Department of Transportation headquarters, 400 Seventh St. SW.

"At that time, General Motors and any other interested parties, including consumers, will be given an opportunity to present testimony, data, and information relating to the initial defect finding," a NHTSA statement said yesterday.

A GM spokesman said that the company "has not heard anything from NHTSA" and will withhold comment until it does.

NHTSA suspects that the defect is caused by an improperly manufactured part, called an end button, in the rear axle shaft and wheel assembly. The button may be too weak, a condition which "may cause excessive axle end play" and a resulting partial or total disengagement of the axle shaft and wheel assembly, NHTSA said.

"Physical separation of the axle and wheel assembly could be a catastrophic event if the vehicle is in motion," NHTSA said.

Among the 2.7 million to 5.3 million cars and sedan pickups affected, NHTSA said it has received reports of 58 cases "where axle shafts with thin end buttons separated completely from the vehicle."

NHTSA said an initial symptom of the suspected condition is excessive rear axle play, which causes a kind of fishtailing motion. Owners experiencing that symptom "may want to ask dealers and knowledgeable mechanics" to check their vehicles, NHTSA said.

The agency urges anyone experiencing the problem to report it in writing to the National Highway Traffic Safety Administration, 400 Seventh St. SW, Washington, D.C., 20590, or by calling the agency's toll-free hotline on 800-424-9393 (426-0123 for the Washington metropolitan area).